This invention relates to an apparatus and method utilizing time domain reflectometry for measuring the condition or characteristics of a material.
Knowledge of level in industrial process tanks or vessels has long been required for safe and cost-effective operation of plants. Many technologies exist for making level measurements. These include buoyancy, capacitance, ultrasonic and microwave radar, to name a few. Recent advantages in micropower impulse radar (MIR), also known as ultra-wide band (UWB) radar, in conjunction with advances in equivalent time sampling (ETS), permit development of low power and low cost time domain reflectometry (TDR) devices. Existing TDR devices are expensive and often impractical for industrial level instrumentation.
In a TDR instrument, a very fast pulse with a rise time of 500 picoseconds, or less, is propagated down a transmission line that serves as a probe in a vessel. The pulse is reflected by a discontinuity caused by a transition between two media. For level measurement, that transition is typically where the air and the material to be measured meet. The amplitude of the reflected signal depends on the difference between the dielectrics of the two media. The dielectric of air is one, while the dielectric of water is about eighty. The larger the difference in dielectric, the larger the reflected signal.
McEwan, U.S. Pat. No. 5,345,471, and other related patents, describe a technique to transmit and receive very fast pulses with simple, low cost and low power electronics. More particularly, McEwan, U.S. Pat. No. 5,609,059, describes a level sensor utilizing this technology. However, the device described therein is intended to be used for simple commercial level applications, such as automobile engine fluid levels. It does not utilize the feature set, power consumption, and versatility required for use in the industrial process environment. Other known devices utilize this technology for a two-wire transmitter using just two wires for both receiving power from the user and sending level information to the user. However, these devices are analog devices limited in the ability to measure the level of extremely low dielectric materials, or materials that coat, clump or build up on the probe, over the wide temperature extremes of industrial process level environments. They also have limited level range capability.
The present invention is directed to overcoming one or more of the problems discussed above, in a novel and simple manner.
In accordance with the invention there is provided a TDR measurement instrument including improved diagnostic and measurement features.
In accordance with one aspect of the invention, there is disclosed a measurement instrument utilizing self diagnostics.
In accordance with another aspect of the invention, there is provided a TDR measurement instrument utilizing automated zoom scaling.
Broadly, there is disclosed herein a time domain reflectometry measurement instrument comprising a probe defining a transmission line. A pulse circuit is connected to the probe for generating pulses on the transmission line and receiving reflected pulses returned on the transmission line, the reflected pulses representing a characteristic of a material being measured. An equivalent time sampling circuit is connected to the pulse circuit for developing an equivalent time representation of the reflected pulses over an expanded time scale within the region of interest. An automatic scale circuit is operatively associated with the sampling circuit for automatically selecting location and size of the region of interest corresponding to characteristics of the probe.
More particularly, the instrument uses a microprocessor that enables added functionality and capabilities. The electronics and probe are tested and calibrated at the factory. Installation and commissioning by the user is simple. The user installs the probe. The transmitter is screwed on the top of the probe. The user connects a standard shielded twisted pair electrical cable to the electronics. Power is applied and the device immediately displays levels. A few simple parameters may need to be entered such as output characteristics and the process material dielectric constant.
Further features and advantages of the invention will be readily apparent from the specification and from the drawing.